The present invention relates to a method of forming a conductor pattern including fine conductor runs on a ceramic substrate, and more particularly to such a method wherein the conductor pattern becomes direct bonded using a eutectic composition to the ceramic substrate.
The technique of direct bonding using a eutectic composition has been used to bond metal such as copper to a ceramic substrate, as discussed, for example, in the following U.S. Pat. No. 3,766,634--G. L. Babcock et al. and U.S. Pat. No. 3,994,430--D. A. Cusano et al., which are both assigned to the same assignee as the present invention and which are incorporated herein by reference. The foregoing bonding technique is also referred to as "direct bonding" in the specification.
A problem has arisen where a metallic sheet or foil such as a copper sheet has been direct bonded to a ceramic substrate. The problem is that gas has been trapped between the copper sheet and the ceramic during the process of direct bonding and caused the metallic sheet to form bubbles or pockets containing the gas. The origin of such gas is not completely understood but it is likely due to entrapment of furnace gas between the metallic sheet and the ceramic during the bonding process, or by emanation of gases from the metallic sheet or from contaminants during the direct bonding process.
One prior approach to overcoming the problem of bubbles occurring in a metallic sheet direct bonded to a ceramic substrate has been to provide an array of circular holes through the metallic sheet. Otherwise trapped gas can evacuate through the holes. This approach, however, is not satisfactory for a metallic sheet which is to be formed into a conductor pattern including fine conductor runs due to the fact that the holes may exist where a fine conductor run is desired.